there is a world of difference between a small block and big block chevy. Yes wieght is something u need to consider depending on the car u build, however the nitro cars that run 4.xxx in the quarter mile at over 300mph dont use small blocks they use 800cid big blocks dont let some ignorant fool confuse u. there is a big difference between the two. Ok to answer ur 1st question the most noticeable difference is torque out put. you 400 sbc may only make 250-300 lb-ft of torque where the same comprssion big block (not taking into account heads cam intakes exhaust and fuel system) will make 350-400 lb-ft of torque. really its not what we prefer its what ur building that matters. I race a 72 chevelle ss that was an origional 402 car. i took the 402 out as fears of blowing up the motor and stuck in a 502 w/ 11:1 comp, aluminum heads big cam and single plane intake with a 1050 carb the car also has some power adders. However i have a college that has a 471 sbc in a 69 camaro with the same compression but set up a little different between exhaust and fuel system and he is always tryin to run me down i really spank him though coming out of the hole.
ASE MASTER TECH drag race and circle track engine builder  

in reguards to u gentlemen discussing the ls series of engines being "loosely" based on the sbc design YOU ARE COMPLETELY WRONG the ls series of engines are far more capable of doing things big blocks could never dream of. The LS engine series is a COMPLETE REDESIGN and is based on nothing general motors has ever done before READ up on the corvette u guys they talk about GMPP will tell u all about it in the OCT 08 hotrod GMPP CRATE ENGINE SHOOT OUT. they took a well setup chevelle and 8 GMPP crate motors and pid them against each other the fastest was a 572 bbc with 700hp but second was LSX 454 with over 100cid less and 100hp less the two motors were less that .500 seconds appart at the track